Modern chemistry's grand challenge is to significantly improve catalysts for water splitting. Further progress requires detailed spectroscopic and computational characterization of catalytic mechanisms. We analyzed one of the most studied homogeneous single-site Ru catalysts, [Ru(II)(bpy)(tpy)H2O](2+) (where bpy = 2,2'-bipyridine, tpy = 2,2';6',2″-terpyridine). Our results reveal that the [Ru(V)(bpy)(tpy)═O](3+) intermediate, reportedly detected in catalytic mixtures as a rate-limiting intermediate in water activation, is not present as such. Using a combination of electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy, we demonstrate that 95% of the Ru complex in the catalytic steady state is of the form [Ru(IV)(bpy)(tpy)═O](2+). [Ru(V)(bpy)(tpy)═O](3+) was not observed, and according to density functional theory (DFT) analysis, it might be thermodynamically inaccessible at our experimental conditions. A reaction product with unique EPR spectrum was detected in reaction mixtures at about 5% and assigned to Ru(III)-peroxo species with (-OOH or -OO- ligands). We also analyzed the [Ru(II)(bpy)(tpy)Cl](+) catalyst precursor and confirmed that this molecule is not a catalyst and its oxidation past Ru(III) state is impeded by a lack of proton-coupled electron transfer. Ru-Cl exchange with water is required to form active catalysts with the Ru-H2O fragment. [Ru(II)(bpy)(tpy)H2O](2+) is the simplest representative of a larger class of water oxidation catalysts with neutral, nitrogen containing heterocycles. We expect this class of catalysts to work mechanistically in a similar fashion via [Ru(IV)(bpy)(tpy)═O](2+) intermediate unless more electronegative (oxygen containing) ligands are introduced in the Ru coordination sphere, allowing the formation of more oxidized Ru(V) intermediate.

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homogeneous phase occur via an "Interaction of 2 M-O units" (I2M) might still be able to carry out the catalytic water oxidation reaction at the surface of an electrode, but will need to proceed through higher energy pathways that can lead to catalyst degradation. 6 Further, given the intrinsic high energy demands for the water oxidation catalysis, it is essential that the anchoring groups that act as an interface between the catalysts and surface are oxidatively resistant.Here on, we report new hybrid materials consisting of molecular WOCs anchored onto Multi-Walled Carbon Nanotubes (MWCNTs) via π-stacking interactions. 9 The resulting materials are extremely stable and allow the anchoring of a large amount of catalyst giving Turnover Numbers (TNs) over a million without apparent deactivation.In a recent publication, 10 we have reported the synthesis of complex {Ru II (tda)(py)2}, 1a, (for a drawing of tda 2-see Scheme 1) and have shown that in its high oxidation states (IV) acts as a precursor for the formation of {Ru V (O)(tda)(py)2} + . The latter is the most powerful molecular water oxidation catalyst described to date achieving Turnover Frequencies (TOF) in the range of 50.000 s -1 . In addition, we showed that the rate determining step for the water oxidation reaction is the O-O bond formation, which in this case occurs via WNA, as evidenced by kinetics and further supported by DFT calculations.Scheme 1. Drawing of the ligands discussed in the present work (top) and complex labelling strategy (bottom).[a]

A molecular water oxidation catalyst based on the copper complex of general formula [(Lpy)Cu II ] 2-, 2 2-, (Lpy is 4-pyrenyl-1,2-phenylenebis(oxamidate) ligand) has been rationally designed and prepared to support a more extended π-conjugation through its structure in contrast with its homologue, the [(L)Cu II ] 2-water oxidation catalyst, 1 2-(L is ophenylenebis(oxamidate)). The catalytic performance of both catalysts has been comparatively studied in homogeneous phase and in heterogeneous phase by π-stacking anchorage to graphene-based electrodes. In the homogeneous system, the electronic perturbation provided by the pyrene functionality translates into a 150 mV lower overpotential for 2 2-respect to 1 2-and an impressive increase in the kcat from 6 s -1 to 128 s -1 . Upon anchorage, π-stacking interactions with the graphene sheets provide further π-delocalization that improves the catalytic performance of both catalysts. In this sense, 2 2-turned out to be the most active catalyst due to the double influence of both the pyrene and the graphene, displaying an overpotential of 538 mV, a kcat of 540 s -1 and producing more than 5300 TONs.Heterogenized water-oxidation catalysis based on earth abundant transition metals, such as Mn, Fe, Co, Ni and Cu, are highly desired for sustainable energy technologies that exploit direct solar water-splitting. 1 An advantage of heterogenized homogeneous catalysts, when compared to heterogeneous catalysts, 2 is that they can be improved by ligand design. Yet first-row transition metal complexes pose several challenges. They usually get deactivated when immobilized on electrode surfaces and they suffer from instability due to hydrolytic behavior and decomposition into metal-oxides upon oxidation of the organic ligands. 3 However, from an engineering perspective, solid-state electroanodes are desired due to the simplicity of assembly for potential devices. Therefore, it is imperative to understand the influence of the anchoring functionality on the performance of the immobilized catalysts to learn how to anchor and stabilize functional molecular catalysts on electrode surfaces. 4,5 Here, we focus on water oxidation by Cu(II) molecular catalysts heterogenized on graphene surfaces.A family of copper complexes based on tetraamide ligands, such as [(L)Cu II ] 2-, 1 2-, (L = o-phenylenebis(oxamidate)) shown in Figure 1, have been recently reported to be effective at catalyzing oxygen evolution by water oxidation at basic pH. 6 Remarkably, the rate determining step (rds) was found to involve reversible oxidation of the phenyl ring. Here, we explore whether the catalytic properties of these complexes can be manipulated by electronic perturbation of the tetraamide π-system, either by modification of the ligand or by π-stacking to graphitic electrode surfaces.

Catalytic O(2) evolution with cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(OH(2))(bpy)(2)](4+) (bpy is 2,2-bipyridine), the so-called blue dimer, the first designed water oxidation catalyst, was monitored by UV-vis, EPR, and X-ray absorption spectroscopy (XAS) with ms time resolution. Two processes were identified, one of which occurs on a time scale of 100 ms to a few seconds and results in oxidation of the catalyst with the formation of an intermediate, here termed [3,4]'. A slower process occurring on the time scale of minutes results in the decay of this intermediate and O(2) evolution. Spectroscopic data suggest that within the fast process there is a short-lived transient intermediate, which is a precursor of [3,4]'. When excess oxidant was used, a highly oxidized form of the blue dimer [4,5] was spectroscopically resolved within the time frame of the fast process. Its structure and electronic state were confirmed by EPR and XAS. As reported earlier, the [3,4]' intermediate likely results from reaction of [4,5] with water. While it is generated under strongly oxidizing conditions, it does not display oxidation of the Ru centers past [3,4] according to EPR and XAS. EXAFS analysis demonstrates a considerably modified ligand environment in [3,4]'. Raman measurements confirmed the presence of the O-O fragment by detecting a new vibration band in [3,4]' that undergoes a 46 cm(-1) shift to lower energy upon (16)O/(18)O exchange. Under the conditions of the experiment at pH 1, the [3,4]' intermediate is the catalytic steady state form of the blue dimer catalyst, suggesting that its oxidation is the rate-limiting step.